Multiple path fading occurs when a radio signal propagates along direct and indirect paths to the receiver. This phenomenon is caused by a variety of atmospheric conditions, such as temperature, humidity or air pressure gradients. In communications systems, fading is undesirable because it produces both amplitude and delay distortion in the received signal; the amplitude distortion being a maximum at the fade notch frequency where the direct path and indirect path signals interfere to the greatest extent. Furthermore, experimental and analytical efforts indicate that fading is a major source of transmission deterioration which, under certain conditions, exceeds system performance objectives.
In a line-of-sight radio system, fading can be represented by a 2-path model. With this model, fading is characterized as being minimum or non-minimum phase fade. With minimum phase fade, the amplitude of the signal propagating over the indirect path is less than the amplitude of the signal propagating over the direct path. Conversely, non-minimum phase fade occurs when the amplitude of the indirect path signal is greater than the amplitude of the direct path signal. It should, of course, be understood that this characterization of fading is a function of time and is basically unpredictable.
It has been found that automatic gain control as well as space diversity techniques are not adequate to maintain satisfactory transmission performance in the presence of fading. Recent efforts have centered on the use of additional equalization to mitigate fading effects and achieve the required level of system performance. Since fading is unpredictable, the compensation provided by a fade equalizer must be capable of automatically adapting for changing signal conditions. One such adaptive equalizer is disclosed by H. Miedema in a U.S. patent application, Ser. No. 158,404, filed June 11, 1980, now U.S. Pat. No. 4,330,764 dated May 18, 1982, and assigned to applicant's assignee. This equalizer compensates for amplitude distortion but does not provide proper delay equalization for non-minimum phase fade. In fact, for non-minimum phase fade the equalizer doubles the delay distortion in the received signal. In another equalizer design disclosed by the applicant in a U.S. patent application, Ser. No. 293,463, filed Aug. 17, 1981, and assigned to applicant's assignee, the equalizer parameters are modified by the character of the multiple path fading. Detecting the character of the fading, however, requires rather complex circuitry.